While Eaves was not in hospital and could be seen congratulating teammates following their 4-2 victory in Game 3, he will be replaced for at least one game by Oleg Saprykin.
"He's a lot better today," Senators coach Bryan Murray said Monday. "He needs a little time, of course.
"A little headache, but beyond that, he's OK."
While the Senators haven't said if Eaves has a concussion, a brain injury caused by force that results in a loss of consciousness is by definition a concussion.
Armstrong wasn't penalized for delivering a hit to Eaves' head with his shoulder as Eaves carried the puck from behind the Penguins' net during the second period.
"I just try to play the game hard and finish my checks," Armstrong said. "It was a play where he came around the net and I just tried to meet (him) at the post and make a hit.
"Hopefully, he's all right. I have to play hard and play in their face. But I didn't mean to pinpoint on his head, I tried to hit the guy."
The play, along with several others during the season, has added fuel to the debate whether any blow to the head should be penalized.
Eaves became the second player to miss playing time with a head injury after being levelled by Armstrong this season. Carolina forward Trevor Letowski missed nine games with a concussion after Armstrong knocked him out with a blindside hit in Pittsburgh on Oct. 14.
Neither Eaves nor Letowski saw Armstrong coming until being hit. Letowski had just made a pass and was skating with his head turned.
While Ottawa forward Jason Spezza said the six-foot-two, 190-pound Armstrong's hit on the six-foot, 192-pound Eaves was dirty, Ottawa coach Bryan Murray called it "a hockey hit."
Murray scouts and prefers to sign players like Armstrong who aren't hesitant to throw their bodies around, even against bigger players. The Senators' willingness to do exactly that has made for a more physical series than expected between two of the NHL's top four scoring teams.
"Guys don't try to bury people in that fashion, you try to knock people down," Murray said. "I don't think Colby Armstrong tried to hit Patty Eaves and knock him out in the game.
"He wanted to get a big hit on him. But I do think something has to be done about guys who get hit in the head, whether it's our guy getting hit in the head or otherwise."
Penguins star Sidney Crosby's answer is to penalize all hits to the head.
"I don't think you can paint every hit with the same brush," Crosby said. "The guys leaving their feet, that's the biggest one.
"When a guy leaves his feet, he's definitely going for the guy's head. But when a guy's bent over, facing you with his head, there's nothing else you can hit."